It is not within the scope of this report to consider all of the ground-based
activities associated with aviation. Overall emissions resulting directly from
aircraft flights do include emissions associated with taxiing and the use of
auxiliary power units at the gate. These emissions are considered briefly here.

8.3.4.1. Auxiliary Power Units

Auxiliary power units (APUs) are engine-driven generators contained in the
aircraft (usually in the tail) that provide the aircraft with necessary energy
during the time the aircraft is at the gate. Part of the generated energy is
used for air conditioning. As an alternative, the required energy can be supplied
by ground-based equipment that delivers electrical power at 400 Hz and preconditioned
air to the aircraft. Herau (1992) investigated the financial and energy savings
of such ground-based installations at Brussels Zaventum Airport's 2000 terminal
and concluded that a significant net saving of carbon emissions could be achieved.
At Zurich Airport in 1997, about 68,000 aircraft used terminal A and B, which
has the facility to provide preconditioned air and 400 Hz power. Provision of
these services achieved estimated savings of about 95% in APU fuel consumption
and emissions. However, fuel used by APUs is only a relatively small part of
the total fuel use of an aircraft. For example, for B737, B747, A310, MD81,
and F100 aircraft, average APU fuel use is only 2.6, 0.8, 1.4, 2.5, and 3.5%
of the fuel use at cruise per hour of operation, respectively (USFAA, 1982,
1995). British Airways estimates that the amount of fuel used by an APU is less
than 1% of the total fuel used by an aircraft.

8.3.4.2. Taxiing

Although standard landing and take-off (LTO) cycles have been used for simplicity,
there are large variations in LTO cycles from airport to airport. Although landing
and take-off times are quite similar for similar fleet mixes, the amount of
taxi/idle varies significantly from airport to airport (Wayson and Bowlby, 1988).
This variance in taxi/idle time is the key factor in the variability of emissions
from aircraft during airport operations (USFAA, 1982, 1988). Various scenarios
have been discussed and attempted to reduce these emissions. These scenarios
include the use of high-speed taxiways, towing of aircraft to runways (Fleuti,
1992), last-minute start-up, improvements in engine design, restriction of certain
aircraft types, realignment of taxiways, improvements at the gate area, and
taxiing in with minimal engines running.

8.3.5. Summary of Other Operational Factors

Aviation passenger mobility efficiency is very dependent on the type of aircraft,
the configuration, the load factor, and the distance flown. Old aircraft use
much more fuel per passenger-km than new aircraft of similar size. The fuel
efficiency of different aircraft is examined in Chapter 7.
The required energy per passenger-km is in the range of 1.0 to 3.0 MJ per passenger-km,
or about 30 to 110 g C per passenger-km. Airlines have generally optimized energy
use per passenger-km, largely because of economic pressures and the requirement
within the industry to minimize operational costs. Thus, with or without environmental
considerations, market and cost considerations are drivers for airlines to optimize
the utilization of an aircraft as much as possible.

CO2 intensity for rail transport also depends on factors such as energy source,
type of locomotive, and load factor, and emissions of CO2 range between < 5
and 50 g C per passenger-km. However, a passenger's choice of mode of transport
is based on fares, total trip time, and frequency-not just environmental considerations.
ECAC (1996) estimates that less than 10% of the European air passenger travel
could be replaced by high-speed train. Yet the scope for substitution is greater
in Europe than in many other parts of the world.

Other operational factors to reduce aircraft fuel burn include optimization
of cruise speed, reduction of tankering, reduction of additional weight, and
energy savings at the airport such as limitations on the use of APUs and reduced
taxi times. The total potential reduction in fuel burn by further optimization
of these factors is in the range of 2-6%. The relative contribution of each
factor is indicated in Table 8-6.

Finally, aircraft noise mitigation measures such as operational changes and
retrofitting of engine equipment on older aircraft to conform with current aircraft
noise standards could have an adverse effect on fuel use. Application of hushkits
could lead to an increase in fuel consumption of up to 5%. However, lightweight
hushkits may have a negligible effect on fuel use.